One critical concern in the fabrication of gas turbine engines is the overall weight of the engine. Excessive weight in the components of the gas turbine engine limits the useful load and reduces the flight range capability (fuel weight) of the aircraft. Thus, it is a goal of the gas turbine engine industry to minimize the overall weight of the gas turbine engine without sacrificing the performance or durability thereof.
It is this effort to minimize the overall weight of the gas turbine engine that has led the industry to the use of hollow fan blades. Each hollow fan blade typically includes a leading edge and a trailing edge extending radially spanwise from a bottom portion to a top portion of the blade. A suction side and a pressure side extend chordwise from the leading edge to the trailing edge. A plurality of hollow cavities is defined within the airfoil between the internal stiffening ribs.
One problem that afflicts the fan blade in general and the hollow blade in particular is foreign object damage. Foreign objects, such as birds and ice in flight and rocks and sand on the ground, are generally ingested with the incoming air into the gas turbine engine. The ingested foreign objects strike the fan blades at high velocities, causing damage from the impact. The impact critical zone on the fan blade is on the leading edge of the blade approximately from sixty percent (60%) to one hundred percent (100%) of the radial span of the blade and up to sixty percent (60%) of the chordwise span.
The damage caused by the impact from the foreign objects may lead to airfoil cracking or in extreme instances to complete engine failure. The hollow airfoils are particularly susceptible to cracks because of the stiffness discontinuity that is inherent in hollow airfoils. The stiffness discontinuity occurs in the transition areas between the stiff solid leading edge and the softer hollow sections of the airfoil. The cracking tends to occur primarily in the areas of greatest stiffness discontinuity.
The current approach for reducing damage in hollow airfoils is to increase the stiffness in the hollow sections of the airfoil. The stiffening of hollow sections is currently achieved by either thickening the side walls between the ribs, thickening the internal ribs, reducing the spacing between the internal ribs, or any combination thereof. However, the above methods of increasing the stiffness of the hollow sections involve a penalty of significantly increasing the weight of each blade. Thus, there is still a need for an optimum balance between the increased impact resistance and the minimized weight of the gas turbine engine hollow fan blades.